This invention relates to inorganic wire insulation for superconducting wire, and more particularly for niobium-tin (Nb.sub.3 Sn) wire.
Fast pulse superconducting magnets for use in airborne AC generators, MHD generators, and energy storage devices require high current, high field, low loss, stable superconductors and associated insulation systems. The multifilamentary Nb.sub.3 Sn superconductor has emerged as the most promising conductor for use in these machines. However, while the electrical properties of multifilamentary Nb.sub.3 Sn, with regard to both high current density and low loss, are advantageous, its mechanical properties lead to other problems. A major concern in developing an insulation system for Nb.sub.3 Sn is the high formation temperature (600.degree.-800.degree. C.) of Nb.sub.3 Sn and the fact that it is a brittle intermetallic compound and after its formation cannot be readily deformed. Its formation reaction at the formation temperature must be carried out when the wire is in its final geometry. An ideal insulation system would, therefore, be one that will not only withstand the breakdown voltage of several hundred volts at liquid He temperature but also withstand the superconductor reaction temperature of 600.degree.-800.degree. C. The insulation should be capable of being applied before reaction, be able to withstand the time-temperature excursion during reaction, and have no adverse effects on the electrical and mechanical properties at low superconducting temperature. In addition, it should provide good thermal contact between the wire and the enthalpy stabilizer, which is usually liquid helium, or in the best case, provide a measure of stabilization itself. This requires that the insulation must withstand this high formation temperature and yet be electrically satisfactory at low use temperature (4-8K). A further requirement of the insulation is that it must absorb the energy dissipated during fast charge and discharge of the magnet. The energy must be absorbed without allowing the temperature of the conductor to rise high enough to quench the magnet.
U.S. patents of interest include U.S. Pat. Nos. 4,407,062 to Sutcliffe et al; 4,261,097 to Weisse; 4,178,677 to Weisse; 3,985,281 to Diepers; and 3,749,811 to Bogner et al. The Sutcliffe et al '062 patent relates to a method of insulating superconductive wire by coating a Nb.sub.3 Sn precursor wire with a layer of a mixture of a silicate of sodium, lithium or potassium and a second component capable of reacting with the silicate to form a ceramic, such as alumina, drying the coating and heating the coating to a temperature in excess of 500.degree. C. to react the silicate and the second component to form the insulating ceramic. The Weisse '097 patent discloses insulating superconductor magnet windings with ceramics, glass, or quartz in the form of filaments, fabrics or nonwoven fabrics. The remaining references are of general interest.